1. Field of the Invention
The present invention generally relates to the general field of propulsion and thrust generation in the low-gravity (orbital) and zero-gravity environments of outer space. More particularly, this invention relates to a system and method for supplying thrust to a satellite or spacecraft for the purpose of performing station keeping and attitude control maneuvers.
2. Description of the Related Art
The capabilities for station keeping and attitude control are of critical importance in nearly all orbital and space-flight systems. Station keeping involves maintaining a desired orbit and/or orientation, despite the influence of forces such as inhomogeneous gravity fields, micrometeorite impacts, and electromagnetic forces. Geosynchronous satellites are an excellent example of station keeping, where their position and alignment are essential to provide uninterrupted service to terrestrial customers. Attitude control generally refers to the control of spacecrafts, such as vessels or ships for transporting humans and cargo, which need to perform maneuvers for various activities such as docking, surveillance, repair or maintenance work, scientific experiments, and re-entry positioning, to name just a few. The ability to provide station keeping and attitude control is therefore seen as an essential element of spaceflight. Such systems may be required to be low-cost, reliable, easily maintained, efficient, fast, environmentally-friendly, and flexible in their design, depending upon the specific mission requirements demanded of them.
The current state of the art in orbital transfers and station keeping involve reaction rockets using a variety of fuels, either chemical (liquid, gaseous, or solid fuel), thermal rockets, or electromagnetic accelerating devices (ion thrusters, magnetohydrodynamics). Chemically-fueled rockets require that fuel be brought along, and this limits the performance of the rocket through the rocket equation.ΔV=Isp·g·In(Mo/Mf)where ΔV is the velocity change given the specific impulse of the rocket motor (Isp), the gravitational constant (g), and the logarithm of the ratio of weights before (Mo) and after (Mf) the burn. Chemical rockets, typically those used for attitude control, may include hazardous chemicals, such as hydrazine, which are highly toxic, and require particular care in fueling, maintenance, and decommissioning. Chemical rockets can be refuelable, though such fuels must typically be produced from terrestrial-based manufacturing facilities. Launching the necessary replacement fuel is quite expensive. Chemical rockets also produce various byproducts, depending on the chemical reaction used to generate the force. These byproducts may be detrimental to the operation of a space vehicle, producing heat, dust, gas, or residues. Chemical rockets also typically involve a number of moving parts and control circuitry which influences their complexity and component count, thereby negatively impacting reliability.
Thermal rockets typically rely on a heat source to impart a high velocity to gaseous atoms, which are directed out a nozzle to generate thrust. A common source of heat is a radioactive isotope, such as strontium. The environmental consequences of a failed launch or a satellite reentry may be unacceptably dangerous. The gaseous atoms are also typically a substance which may be abundant on Earth but rare in space, therefore requiring the expense of launching replacement fuel.
Electromagnetically accelerated rockets, such as the electrostatic ion engine, typically use some gaseous fuel which is ionized and then accelerated to produce thrust.
Engines of this type are efficient, however, their specific impulse is generally quite low and they are therefore of limited use in space mission applications. More powerful methods of electromagnetic rocket engines have been proposed, however, these are more typically aimed at providing main thrust and not necessarily station keeping or attitude control.
In view of the above, existing methods of station keeping and attitude control suffer from a variety of drawbacks. It would therefore be desirable if improved performance of station keeping and attitude control system could be achieved with the convenience of freedom from terrestrial fuel sources and simple refueling using materials commonly found in space environments.